The present invention relates to plasma torches and, more particularly, high-power plasma torches for which the life span of at least one of the electrodes is prolonged.
Plasma torches or arc plasma blowlamps are known in technology. This type of torch consists essentially of two tubular and coaxial electrodes, one upstream and one downstream as identified with respect to the direction of outflow of the plasma, which are separated by a chamber. An arc is established between the electrodes and, simultaneously, a plasma-generating gas is injected into the chamber which separates the electrodes. The arc which is struck between the electrodes is maintained and carries the gas at very high temperature and ionises it. At the outlet of one of the electrodes, the downstream electrode, this gas is driven at a high speed and the plasma which it constitutes forms the heat-carrying agent.
Certain types of plasma torch deliver powers lying between 100 and 500 kW and those to which the invention applies more particularly may produce several megawatts as is necessary for certain industrial, for example metallurgical, applications.
In this type of plasma torch, the electrodes are consumable components. The life span of the electrodes is a function of numerous parameters. Contributory factors are, for example, the power of the torch and, more particularly, the value of the arc current, the nature of the plasma-generating gas injected, since, due to its decomposition, reactions may take place with the constituent materials of the electrodes. The life span of the electrodes is, also, the function of the tasks performed by the torch, according to whether they are continuous or discontinuous.
The life span of the electrodes may vary from a few tens of hours for relatively low-power torches, to several hundreds of hours for those of high power to which the invention more particularly relates.
This relatively short life span of the electrodes is a significant drawback, particularly in an industrial context.
In order to attempt to remedy this drawback, it has been proposed to equip this type of torch with at least one magnetic field coil which locally surrounds preferably the upstream electrode, and to supply the latter with power with the aid of means which make it possible to control the displacement of the upstream foot of the arc on the upstream electrode in such a way as to make it describe an alternating longitudinal course, on which is superimposed preferably an oscillation or a vibration of the arc foot during the sweeping proper.
One solution of this type is, for example, disclosed by the document FR 2 609 358. According to the solution provided by this document, the field coil which locally surrounds the upstream electrode is supplied with power with the aid of a particular electrical circuit, which is specific to it and which is supplied with variable direct current whose level changes in steps or progressively and whose level furthermore preferably has a pulsating ripple whose frequency is significantly higher than that of the variation in the direct current on which it is superimposed. It can be imagined that the use of a special, independent, electrical power supply circuit for the field coil which is added to the main power supply circuit necessary for striking and maintaining the arc proper, technically complicates and financially increases the cost of the installation,